The conditions leading to the Donnan membrane equilibria arise from the inability of ions to diffuse out from one phase into the other in a heterogeneous system. This fundamental principle, originally introduced during the early years of twentieth century and popularly known as Gibbs-Donnan equation or Donnan membrane principle, can be judiciously exploited to formulate the preparation of a new class of hybrid sorbents. Ongoing research activities at Lehigh University have demonstrated that by appropriately dispersing metal oxide nanoparticles within the gel phase of polymeric ion exchangers, their sorption capacities can be i) greatly enhanced due to counter-ion inclusion or ii) practically reduced to zero due to Donnan co-ion exclusion effect. These metal oxides, namely, Fe(III), Zr(IV), Al(III), Ti(IV) oxides and magnetite are environmentally benign and inexpensive. Such fluctuations in capacity result from the Donnan membrane effect exerted by the ion exchanger support media. In our earlier work on arsenic removal, we developed the first hybrid polymer-based regenerable and reusable arsenic-selective sorbent by dispersing hydrated iron oxide nanoparticles within the gel phase of anion exchangers. Very recently, we successfully dispersed amorphous zirconium oxide and iron oxide nanoparticles within thin cylindrical ion exchange fibers (IX-F) offering fast sorption/desorption kinetics.